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C. Soil Organic Matter Build-Up

C. Soil Organic Matter Build-Up

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fertility and integrity is much more SOM dependent. In some tropical

Brazilian soils, 70–95% of cation exchange capacity (CEC) is founded in

SOM (Bayer and Mielniczuk, 1999). In such soils, SOM maintenance or

build‐up is crucial to ensuring good crop productivity, and is often postulated as the single most important element of the soil restoration process

associated with Brazilian zero‐till regimes. In principle, both decreased

erosive losses of SOM‐rich topsoil (Lal, 2002; Rasmussen and Collins,

1991) and slower SOM mineralization rates in zero‐till soil compared to

plowed soil suggest that zero‐till may provide more favorable conditions for

SOM build‐up than conventional tillage. Not turning the soil, for example,

means that: (1) less soil macroaggregates are disrupted, consequently leading

to the increased formation of stable microaggragates that occlude and

protect particulate organic matter (POM) from microbial attack (Amado

et al., 2006; Feller and Beare, 1997; Lal et al., 1999; Six et al., 1998, 1999,

2000), that (2) there is less stimulation of short‐term microbial activity and

concomitant release of CO2 in response to enhanced soil aeration (Bayer

et al., 2000a,b; Bernoux et al., 2006; Kladivko, 2001), and that (3) there is

less mixing of residues deeper into the soil where conditions for decomposition are often more favorable than on the soil surface (Blevins and Frye,

1993; Karlen and Cambardella, 1996). In this context, Mielniczuk (2003)

estimated the rate of SOM mineralization under conventional tillage regimes

in Southern Brazil to be on average 5–6% per year compared to an average

of about 3% per year in zero‐till soils.

Although the actual amount of SOM storage potential in a given soil is in

turn largely determined by climate and the capability of soils to stabilize and

protect SOM, this itself generally being largely determined by soil texture,

soil mineral surface area, and soil mineralogy, with soil parameters such as

water‐holding capacity, pH, and porosity acting as rate modifiers (Baldock

and Skjemstad, 2000; Six et al., 2002b), the large majority of Brazilian

literature does indeed suggest that SOM accumulation in zero‐till soils

above that of plowed soils occurs, and that this is the case over a range of

soil textures, from sandy loams (Amado et al., 1999, 2000, 2001, 2002, 2006;

Bayer et al., 2000a,b, 2002) to heavy clay (>60% clay) soils (Amado et al.,

2006; De Maria et al., 1999; Perrin, 2003), both in Southern Brazil (Muzilli,

1983; Sa´ et al., 2001a,b; Zotarelli et al., 2003), as well as in the cerrado region

(Corazza et al., 1999; Freitas et al., 1999; Resck et al., 1991, 2000; Scopel

et al., 2003). Bernoux et al. (2006) recently reviewed some 25 published and

unpublished data sets on the rate of C (SOM $58% C) accumulation in

Brazilian zero‐till soils and observed that reported C accumulation rates in

excess of those found in comparable plowed soils vary from around 0.4–1.7

t C haÀ1 yearÀ1 for the 0‐ to 40‐cm soil layer in the cerrado region and

between À0.5 and 0.9 t C haÀ1 yearÀ1 in Southern Brazil. They further noted

that average rates of C storage amounted to about 0.6–0.7 t of C haÀ1 yearÀ1



in all reported regions of Brazil when the soil surface layer was considered

(0–20 cm), although these values combine diVerent soil and crop types, and

the actual site‐to‐site/experiment‐to‐experiment variation was high. We

found over 40 published articles relating to SOM dynamics in Brazilian

zero‐till regimes (with very few exceptions all from experimental stations

or trial plots rather than farmers’ fields), but reviewing them in more detail

reveals a varied picture, which is compounded by the fact that relevant

reports originate from various climates and soils with diverse tillage, cropping and fertility management histories, as well as often being sampled to

diVerent depths and based on analytical and calculation methods of varying

accuracy. Freitas et al. (1999), for example, observed increases in SOM in

coarse particle‐size fractions (200–2000 mm) down to 20‐cm depth compared

to similarly cropped but plowed land in a clayey cerrado Oxisol already after

only 4 years of zero‐till, while other work reported a decrease in SOM

compared to plowed soil down to a depth of 10 cm after 3 years in a Oxisol

in Toledo (Riezebos and Loerts, 1998), to a depth of 20 cm after 11 years of

zero‐till in an Oxisol in Passo Fundo (Machado and Silva, 2001), to a depth

of 40 cm after 10 or 22 years of zero‐till in either a well drained, Typic

Hapludox Oxisol in Tibagi (Sa´ et al., 2001a) or an Oxisol in Londrina

(Machado and Silva, 2001), respectively, and Sisti et al. (2004) and Castro

Filho et al. (2002) found no significant increase in SOM down to 30‐cm

depth in a clayey Typic Hapludox Oxisol after 13 years of zero‐till in Passo

Fundo or down to 40‐cm depth even after 21 years of zero‐till in a Typic

Haplorthox Oxisol in Londrina, respectively.

Sampling depth is an important issue in terms of SOM accumulation

studies in Brazil, and results are strongly influenced by the pattern of SOM

storage. In the absence of soil inversion and mixing, zero‐till soils have

highly stratified SOM stocks, SOM being most concentrated near the surface

and gradually decreasing with depth (Machado and Silva, 2001; Sa´ et al.,

2001a,b). Additionally, and importantly in this context it is essential to note

that direct comparisons in absolute SOM storage between plowed and zero‐

till soils are inappropriate if soil depths less than 20 cm are considered, as

conventional soil tillage homogenizes SOM down to 20 cm (Bernoux et al.,

2006; Reicosky et al., 1995). Deeper samples, however, also show diVerent

trends. Studies performed in the cerrado region by Centurion et al. (1985)

and Corazza et al. (1999) showed that while soil C stocks under zero‐till were

higher than under plowed soils in the surface 20 or 30 cm, extending

sampling depth to 100 cm evened out global diVerences in SOM between

tillage systems due to lower C content under ZT in the 30‐ to 100‐cm depth

interval. Sisti et al. (2004), on the other hand, found much larger diVerences

in total SOM between zero‐till and plowed soil if soils were sampled down to

a depth of 100 cm, the 30‐ to 100‐cm depth interval containing between 50%

and 70% of the extra C in zero‐till compared to tilled soil. This, they



reasoned, could possibly be explained by the greater root density at depth

under zero‐till compared to the plowed soil in their study, while the acidic

subsoil in the studies by Centurion et al. (1985) and Corazza et al. (1999)

in the cerrado region may have inhibited much rooting at depth. We found

no other Brazilian literature that reports SOM storage at depths greater than

40 cm.

Brazilian research data also indicate that the pattern and quality of SOM

in zero‐till soils diVers to that of plowed soils. Various research has also

found that the relative amount of free labile or more recent (e.g., POM)

rather than humified and occluded SOM fractions is higher in zero‐till soils

compared to plowed soils, which is in turn has important ramifications for

soil structure, nutrient cycling and as a source of energy for soil microbial

biomass. Using a particle‐size fractionation technique combined with electron spin resonance, Bayer et al. (2000b), for example, observed that soil

organic C (SOC) associated with sand and silt fractions in zero‐till soils was

less humified and therefore younger than that associated with finer fractions,

while Sa´ et al. (2001a) reported that although they also found higher SOC

concentrations in the finer particle‐size fractions (<20 mm) under zero‐till

compared to conventional tillage, the percentage of SOC derived from crop

residues, as assessed by 13C natural abundance, was generally greater in the

coarse (>20 mm) fractions than in the finer ones. Similarly, Amado et al.

(2006), investigating SOM storage in four long‐term trials in a range of light

(<9% clay) to heavy (>70% clay) soils in Southern Brazil, noted that free

light fraction SOM was on average 3.5 times higher under zero‐till than in

tilled soil, stipulating that this was probably a consequence of lower soil

temperatures and residue‐soil contact in zero‐till soils compared to plowed

soils. They therefore conclude that physical protection of SOM was important in zero‐till, especially in sandy soils, but that in contrast to neighboring

soils under native vegetation, soil texture played a less important role in

short‐term SOM stabilization.

Finally, and most importantly in terms of actually managing SOM build‐

up, the increase in plant biomass per unit of land and time through fast‐

growing cover crops typical for Brazilian zero‐till systems means that more

fresh organic matter is added to soils than under traditional double‐cropping

plow regimes. Although physicochemical characteristics inherent to diVerent

soil may partially limit increase in SOM with increased organic inputs,

various studies suggest that SOM responds linearly to increasing rates of

residue addition over a variety of soils and climates (Bayer, 1996; Black,

1973; Burle et al., 1997; Rasmussen and Collins, 1991; Testa et al., 1992;

Teixeira et al., 1994). Burle et al. (1997), for example, obtained a close

relationship between SOC in the 0‐ to 17.5‐cm soil layer and residue quantity

added by 10 diVerent zero‐till cropping systems. Results obtained by Bayer

(1996) stipulated that after 9 years of zero‐till with high‐residue addition



(14 t of dry matter per ha per year), SOC content increased by 11 t haÀ1 in

the top 17.5 cm of soil compared to conventional cropping systems that only

yielded additions of 6.5 t of residues haÀ1 yearÀ1. In Southern and tropical

Brazil, with average annual temperatures close to or above 20 C and concomitantly high‐decomposition rates, it is therefore probably necessary to

annually add between 8 and 10 t of residue dry mass haÀ1 yearÀ1 in order to

maintain the SOM stocks under zero‐till, which, as Mielniczuk (2003) postulates is only really feasible if both suitable high‐biomass cover crops and

main crops are resorted to. Especially fodder grasses, as used as dry season

cover crops in the cerrado (Section III.B.2), but also cereal cover crops, such

as millet, sorghum, and black oat, or crucifers, such as oilseed radish and

legumes such as vetches and lupines (Calegari, 1998c), can produce high

amounts of biomass (Table III), and in combination with high‐residue

producing main crops, such as maize rather than soybean or wheat, can

help boost SOM stocks. Importantly also in this context, it is essential to

consider below‐ground biomass of crops and cover crops, especially as the C

added in roots can be close to double that of shoots (Mielniczuk, 2003) and,

as discussed earlier, Sisti et al. (2004) found increased roots under zero‐till

compared to plowed soils well below plowing depth.

De Maria et al. (1999) compared SOM stocks after 9 years of either maize

or soybean in summer and oat in winter both under zero‐ and conventional

tillage. Despite the fact that maize produced much larger biomass quantities,

these did not increase SOM under either tillage type, and SOM actually

decreased over the study period in all treatments. As the net N balance

(fertilizer N – grain N export) was only about 20 kg of N per hectare, the

lack of SOM accumulation under either tillage treatments may be related to

the lack of suYcient external N input to the system. Sisti et al. (2004) and

Amado et al. (2006) further studied the role of N additions in SOM build‐up

under zero‐till in Brazil, and both found that where rotations with N2‐fixing

legumes were included, much more SOM was accumulated, hence highlighting the fact that for there to be an accumulation of SOM there must be not

only a C input from crop residues, but a net external input of N. Sisti et al.

(2004) observed that where hairy vetch was planted as a winter cover crop in

rotations that included common oat and wheat in winter and maize or

soybean in summer, soil C stocks were increased by approximately 10 t haÀ1

down to a depth of 100 cm after 13 years of zero‐till, soil C being 17 t higher

in this soil layer than in comparable plowed soils. They further postulated

that where net N balance was close to zero over the whole crop rotation,

little SOM accumulation was to be expected. Amado et al. (2006) reported

that pigeonpea and mucuna cover crops integrated into zero‐till maize

cropping systems had the highest C accumulation rates under zero‐till and

that intensive cropping systems, including mixes of black oat with hairy

vetch in winter and maize with cowpea in summer, as well as rotations of



oilseed radish and ryegrass, among other crops, eVectively increased zero‐till

C accumulation rates compared to more conventional double‐crop systems

used by many farmers.

In summary, a large body of Brazilian work corroborates the fact that

SOM accumulates under zero‐till in excess of that under plowed land, and

that farmers can in theory influence SOM build‐up through astute crop

rotations and appropriate liming and fertilization regimes. The amount

and rate of SOM build‐up is, however, less clear. This complexity of data

on Brazilian SOM accumulation make it hard to draw any firm conclusions

about a possible timeframe for which SOM levels significantly increase.

However, Six et al. (2002a), reviewing literature on SOM dynamics in

tropical and temperate zero‐till soils around the world, remarked that

there commonly is a relative increase in SOM in the upper 40 cm of zero‐

till soil after 6–8 years when compared to tilled systems under similar

cropping regimes, and this pattern could potentially hold true for a large

amount of the Brazilian data.


Possibly one of the most important immediate nutrient eVects of zero‐till

is the potential of the residue cover to restrict N availability. Residues with a

high C to N ratio, such as black oat, wheat, maize, sorghum, and ryegrass,

commonly induce N immobilization in soil surface strata during decomposition, although the magnitude of this eVect is dependent on residue quantity

and quality, as well as the mineral status of the soil. Sa´ (1999) suggests that

the immobilization process is most intense during the first years of zero‐till,

but after 5 or more years, gradually diminishes due to the increased surface

concentration of SOM acting as an N source and thereby eVectively counteracting N limitations induced by residues. Especially as zero‐till increases the

POM stock, which is strongly correlated to potentially mineralizable N, soil

N availability under long‐term zero‐till is suggested to increase over time

when compared to conventional tillage (Sa´ et al., 2001a,b). During the first

few years of zero‐till, however, cereal response to N fertilization is high,

and generally N‐fertilization is recommendable or necessary (Calegari,

2002), although care should be taken to distance fertilizer deposition as far

as possible from the mulch in order to avoid fertilizer immobilization

(Wiethoălter, 2002).

An option of adding N to the system and alleviating immobilization‐

induced N constraints under zero‐till is by rotating legumes with nonlegumes, as well as mixing legume swards with nonlegume stands. Residues

with a low C to N ratio as common vetch, lupine species, soybean, oilseed

radish, mucuna, jackbean (Canavalia ensiformis L. DC.), or pigeonpea can



increase N availability. Much work has been conducted on the nutrient

content and residual eVects of common cover crops in Southern Brazil,

Paraguay, and Uruguay (Amado et al., 1990b; Calegari, 1989, 1990, 1995;

Calegari et al., 1993; Derpsch and Calegari, 1992; Derpsch and

Florentin, 1992; Igue, 1984; Jucksch et al., 1984; Kage, 1984; Lovadini

et al., 1972; Monegat, 1991). Relating to the results of trials conducted

over 2 years at IAPAR in Londrina, Derpsch et al. (1986), for example,

reported that maize fertilized with P and potassium (K) produced highest

yields after preceding crops of white lupine and hairy vetch, when compared

to yields after grasspea (Lathyrus sativus L.), cereals, and sunflower. This can

be explained by the fact that appropriate legume residues can decrease maize

mineral N requirement by about 60–90 kg haÀ1 (Amado et al., 2000;

Calegari, 1995; Sa´, 1999). Both in tropical and subtropical Brazil, legume

residues left on the soil surface decompose rapidly and provide a prompt N

release, sometimes so fast that it causes asynchronies with maize demand

(Acosta, 2005; Giacomini, 2001; Vinther, 2004). Common vetch residue left

on soil surface in Santa Maria, for example, released 60 kg of N per hectare

in only 15 days (Acosta, 2005). Derpsch et al. (1986) also noted high‐maize

yields after oilseed radish in the same trials, which they explained as a

consequence of the high amount of N (135 kg haÀ1) in the plant shoots

and roots at harvesting time. Muzilli et al. (1983), Calegari (1985), Amado

et al. (1990b), Derpsch and Calegari (1992), and Debarba and Amado (1997)

all also reported positive eVects of N supply provided by legume cover crops

such as white lupine and hairy vetch in Southern Brazil, while Carvalho et al.

(1996) noted that pigeonpea and sunnhemp fulfilled a similar function in the

cerrado region. Sisti et al. (2001) actually found that legumes grown under

zero‐till symbiotically fixed a higher proportion of their N requirements

compared to legumes sown to plowed soil, which is presumably a consequence of the lower rates of N mineralization and concomitant higher

dependency on fixation when soils were not turned. Acosta (2005), using


N labeling, found that common vetch symbiotically fixed 50–90% of its N

requirement in a zero‐till trial in Santa Maria, Rio Grande do Sul. Burle

et al. (1997) trialed mixed stands of cover crops over 10 years and found

maize unfertilized with N to respond best to a preceding mixture of black

oats and hairy vetch compared to nine other cover crop combinations

planted prior to maize (Table IV). This is most probably a result of the

maize profiting both from the beneficial eVects of lasting soil cover and large

C inputs generated and gradually laid down by the fast‐growing oat, as well

as the symbiotically fixed N from the vetch. Giacomini et al. (2003) also

found mixtures of black oats or oilseed radish and hairy vetch to be the most

eYcient way of combining both physical soil protection through long‐lasting

residues and high‐biomass production with N fixation in Southern Brazilian

zero‐till systems.



Table IV

Maize Yields on a Zero‐Tilled Oxisol in Southern Brazil After 10 Years of One of Seven

Cropping Regimes and Fertilized with Either 0 kg haÀ1 N or 120 kg haÀ1 N a

Cropping systems studied


Avena strigosa

A. strigosa ỵ Vicia sativa

A. strigosa ỵ Trifolium


Macroptilium atropurpeum

(8 years)

Cajanus cajan


Digitaria decumbens (8 years)

Grain yields (t haÀ1)


0 kg ha1 N

120 kg ha1 N


Maize ỵ Vigna unguiculata


2.0 a A

6.6 b B

5.4 b B

7.1 B

7.6 B

7.0 B

Maize (5th and 10th year)

5.7 b A

8.3 B

Maize ỵ C. cajan


Maize (5th and 10th year)

5.4 b B

1.1 a A

1.3 a A

7.2 B

6.5 B

6.8 B


Means followed by the same small letter down rows or capital letter across columns are not

significantly diVerent using the Tuckey test at p ¼ 0.05 (data from Burle et al., 1997).

For zero‐till maize production in Southern Brazil, variations in traditional

mineral N fertilization regimes have also been tested. The use of part of

maize N fertilization in the black oat cover crop had a positive eVect in terms

of increasing black oat residue quantity and quality (lower C:N ratio), but

this in turn had a fairly limited eVect on N supply to the following maize

crop (Amado et al., 2003). Another zero‐till fertilizer strategy is to use the

total rate of mineral N at cover crop termination (approximately 15 days

before the seeding the main crop) or at maize seeding time rather than apply

N in split applications, assuming that the residue mulch will temporarily

bind added N and thereby partially prevent leaching losses of N, as this

eliminates the need for an additional field operation. However, in terms of

maize yields, this strategy was only eYcient in years with light rainfall

during maize growth (Basso and Ceretta, 2000; Poăttker and Wiethoălter,

2002; Sa´, 1999). In years with high rainfall, the traditional strategy of

applying one‐third of N at seeding and the remaining two‐thirds as a top

dressing after 6 weeks was more eYcient (Ceretta et al., 2002; Poăttker and

Wiethoălter, 2002).

As N fertilizer is not thoroughly mixed into the soil, concerns about N

volatization in zero‐till are frequent (Blevins and Frye, 1993). Cabezas et al.

(1997), for example, evaluated the eYciency of broadcasting of urea, the

most common mineral fertilizer source of N in Brazil, on mulch, and found

that about 80% of N was lost through volatilization. In this context, however,

we would like to stress that this result was obtained under hot and dry

conditions common in the cerrado, and under wet winter conditions in

Southern Brazil, Wiethoălter (2002) found that only about 5% of broadcast



urea‐N was lost by volatilization in a wheat crop. The discrepancy between

the results most likely is explained by the diVerences in weather conditions.

A light rain after broadcast urea in zero‐till can reduce the N volatilization.

As with N, P has been shown to accumulate in soil surface strata under

zero‐till regimes (Sa´, 1999), due to the management eVect of broadcasting or

row applying P fertilizer rather than incorporating it, but also due to

decomposition of P‐containing residues on the soil surface and the slow

movement of P through the soil profile. As this corresponds to soil strata

that is richest in SOM under ‘‘mature’’ zero‐till, P phytoavailability has been

shown to improve, due both to lower P sorption on clay particles and

iron and aluminum sesquioxides surface, as well as due to increased

biological activity in this strata (Afif et al., 1995; De Maria and Castro,

1993; Falleiro et al., 2003; Fontes et al., 1992; Lopes et al., 2004; Muzilli,

1985; Reinert, 1982; Sa´, 1999; Selles et al., 1997; Sidiras and Pavan, 1985;

Silva et al., 1997). This eVect is exacerbated by the fact that there is generally

also a higher concentration of surface roots under zero‐till compared to

plowed soils (Holanda et al., 1998; Stone and da Silveira, 1999). Gassen

and Gassen (1996), for example, reported that after some years, demand for

fertilizer P is up to 50% lower under zero‐till compared to crops with the

same P uptake in plowed soil. Furthermore, Sa´ (1999) found that organic P

made up 70% of total P in the 0‐ to 20‐cm soil strata under zero‐till,

consequently suggesting that organic P in zero‐till could play an important

role in maintenance of the P equilibrium in the soil solution as it is more

mobile than inorganic P. Lopes et al. (2004), however, only found organic P

in an Oxisol to amount to 25–35% of total P, and Oliveira et al. (2002) argue

that, similarly to N, P in the first years of zero‐till is immobilized in the

organic matter that is being built up, the SOM therefore acting as a temporary P drain before the beneficial eVects of SOM on P phytoavailability

become evident.

Similar to N and P, but also calcium (Ca) and magnesium (Mg) (Calegari,

2002), K has a higher concentration in topsoil (0–10 cm) than in deeper soil

layers under zero‐till, but, due to its high‐soil mobility, its stratification is not

as extreme as that of P. The significant increase of CEC as a result of

increased surface SOM concentrations under zero‐till, especially in low

activity or sandy soils, has also been shown to greatly reduce K leaching

under zero‐till compared to plowed soil (Bayer, 1996). Furthermore, crop

rotations have proved particularly important in the maintenance of K under

zero‐till, with cover crops, such as millet and oilseed radish oil having a high

potential to absorb and hence recycle K, hence functioning as K catch crops

and thereby reducing K leaching losses. In the cerrado region, where many

soils have low K, the combined use of K fertilizers and cover crops with zero‐

till showed an increase in soil K above that of conventional tillage (Lopes

et al., 2004).





Soil compaction is another contentious issue in Brazilian zero‐till, which,

if severe, can potentially both curtail infiltration rates, as well as restricting

crop root development, which in turn is especially important in areas prone

to periods of in‐season drought and poor soil fertility. While soil compaction

is a natural process, the cohesion between aggregates tending to decrease

under the flux of water in soil, this process is counterbalanced in natural

ecosystems by intensive biological activity throughout the soil profile. In

cultivated land, on the other hand, compaction is often accentuated by the

random traYc of machinery, animals, or humans (McGarry, 2003). Plowing

is in turn commonly used to remedy compacted soil, at least to 20‐cm depth,

while in Brazilian zero‐till, activating and enhancing biological activity is the

key to avoiding natural or man‐induced compaction, as is the case in natural

ecosystems (Scopel et al., 2003). In theory, bulk density may well increase

over time under zero‐till, but infiltration rates remain reasonable due more

favorable porosity, pores being continuous and vertical, postulate McGarry

(2003) and Scopel and Findeling (2001). Farmer experiences with soil compaction published in the Brazilian literature, however, are somewhat

conflicting. Sa´ (2000) gives soil compaction and resulting yield declines,

especially during dry periods, as a reason for some Southern Brazilian

farmers to abandon zero‐till and returning to conventional cultivation.

Ribeiro et al. (2005), relying on data from a survey of 60 farmers in southern

Parana´, elaborate on this, maintaining that farmers justify the use of chisel

ploughs for soil decompaction or for breaking soil crusts, the latter occurring specially in soils with high amounts of silt. Conversely, however,

Derpsch (2001) argues that compaction does not translate in reduced soybean yields, while researchers in Rio Grande do Sul further reason that soil

compaction there is not big issue, despite the high sand and silt content

making soils very prone to compaction, because suitable planter‐rippers

are able to break the shallow soil compaction induced by cows over the

winter period.

In more detail in terms of experimental trials, Derpsch et al. (1986) found

that after 7 years of zero‐till, bulk density at 0‐ to 20‐cm depth in a clayey

Oxisol in Southern Brazil was greater than under conventional tillage,

whereas the plowed soils had more or less pronounced ‘‘plow pans’’ at 20‐

to 30‐cm depth. Furthermore, total and macropore volume was considerably

lower under zero‐till, while mesopore space was higher and micropore space

unchanged between tillage systems. Corsini and Ferraudo (1999), on the

other hand, found that although during the first 3 years of zero‐till on a clayey

Oxisol soil macroporosity and root development was lower under zero‐till

than in an adjacent tilled plot, the long‐term benefits of continuous zero‐till on

soil macroporosity kicked in during the fourth year. After this, macroporosity



and root development values increased and matched corresponding levels

of freshly plowed soils in the experimental area during the eighth year.

Similarly, Machado and Silva (2001) and Oliveira et al. (2003) remarked

that bulk density at 0‐ to 20‐cm depth after 11 or 20 years of zero‐till on an

Oxisol was not greater than bulk density under conventional tillage on the

same soil. At 20–30 cm, however, bulk density of the plowed soil was greater

than of the zero‐till soil.

Part of the relative ‘‘ecompaction’’ process is undoubtedly due to the

eVects of gradual increases of SOM on soil structure and integrity. SOM

has a direct impact on soil bulk density (or inversely on the porosity), both

because the particle density of organic matter is considerably lower than that

of mineral soil, but also because SOM is often associated with increased

aggregation and permanent pore development as a result of increased soil

biological activity (Franzluebbers, 2002). Various Brazilian authors have

pointed at the beneficial influence of increased surface SOM levels under

zero‐till on soil structural stability and aggregate size and stability (Campos

et al., 1995; Carpenedo and Mielniczuk, 1990; Castro Filho et al., 1998;

Silva et al., 2000), although the degradation and the opposite process—

restoration of structural stability under zero‐till—have been shown to be

very dependent on soil texture and are much faster in sandy soils than in

clayey soils. Borges et al. (1997), for example, observed that zero‐till on

sandy (>70% sand) soil restored water aggregate stability to near 70% of

original levels of undisturbed soil after 3 years, whereas Da Ro´s et al. (1996)

found that in clayey soil, where SOM storage potential was much greater,

similar values were only achieved after 9 years of zero‐till. Castro Filho et al.

(1998) further reported that soil aggregation had a tendency to increase

when crop rotations included plant species such as maize, whose residues

had high C to N ratios. Roth et al. (1988) concluded that even though porosity

was lower in soils after 7 years of zero‐till compared to tilled soil, this

was oVset by a higher aggregate stability under zero‐till, so that in the end

no significant diVerences in infiltrability were found between tilled

and untilled soils, even when neither soils were covered by adequate amounts

of residues.

Additionally to the eVect on soil aggregation, the increase in SOM in

surface layers under zero‐till may also aVect plant‐available moisture

levels, as SOM has a greater water‐holding capacity than mineral soil

(Franzluebbers, 2002), and even if roots growth is restricted, this may be

compensated by the fact that roots need to explore less soil volume to get

water. Another explanation for the absence of yield decreases even in soils of

high bulk density is that, as put forward earlier in this section, roots and

water may exploit continuous biopores and channels generated by previous

plants or soil fauna (Ehlers, 1975). Using suitable crops to break through

compacted soil layers and create biopores is a feasible strategy to circumvent



compaction problems. Kemper and Derpsch (1981) argue that crop rotations involving deep‐rooted cover crops, such as hairy vetch, sunflower,

castor bean (Ricinus communis), pigeonpea, or oilseed radish, may promote

biological loosening of compact soils. Machado and Silva (2001) showed

that if hairy vetch and maize were included in zero‐till rotations of soybean

and wheat, soil bulk density actually tended to be lower than in plots only

cultivated with soybean and wheat. Especially a bulbless variety of oilseed

radish is often reported as an outstanding example of biological plowing

in Brazil, while Se´guy et al. (2003) maintain that plant species, such as

Brachiaria, Eleusine, or Cynodon species, are very eYcient in restoring the

soil structure both thanks the abundance of roots they develop in the first

0–40 cm of soil, as well as their overall strong root systems.


The control of soil acidity is often viewed as one of the most controversial

aspects of eVective zero‐till. Due to the absence of soil inversion under zero‐

till systems, applied sources of lime are not physically mixed into deeper soil

strata, and diVerent approaches are required in order to tackle soil acidity

problems. The most conventional approach is to rectify soil acidity before

commencing zero‐till, and Derpsch (2001) and Aghinoni (1989) recommend

applying lime the year before entering into zero‐till, thereby making use of

the opportunity to incorporate lime. In general, if crop residues are thereafter returned to the soil, acidification should not present a problem due to

the decarboxylization of organic anions, ligand exchange, and the addition

or retention of basic cations (Miyazwa et al., 1993; Yan et al., 1996).

Research by Kretzschmar et al. (1991), for example, showed that millet

straw left on fields increased pH from 4.5 to 5.7 over 6 years. Long‐term

tillage and crop rotation experiments on acidic soils in Brazil have indicated

that zero‐till may increase pH, KCl‐exchangeable Ca and Mg, and Mehlich‐1 P,

and decreased KCl‐exchangeable Al (Calegari, 1995; Calegari and Pavan,

1995; Sidiras and Pavan, 1985) compared to conventional tillage (Machado

and Gerzabek, 1993; Muzilli, 1983; Sidiras and Pavan, 1985).

Another approach is to broadcast lime or dolomite on the soil surface and

allow it time to leach (Caı´res et al., 1996; Lopes et al., 2004). Work by Sa´

(1993) indicated that surface application of lime after 270 days was superior

to its incorporation to 20‐cm depth with zero‐till on distrophic red‐yellow

and dark red Oxisols in Parana´, while Lopes et al. (2004) agree that when the

level of soil P is satisfactory, it is possible to achieve highly productive

cultures in zero‐till soils by applying calcareous material to the soil surface

without incorporation, the quantity of material needed for this being lower

than when the material is incorporated into the soil, although the maximum

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C. Soil Organic Matter Build-Up

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